The invention relates to an apparatus and a method for modulation of an optical signal having an optimized modulation format.
Multilevel modulation formats have recently received more attention, as they allow to transport a larger amount of data with a smaller bandwidth than conventional binary modulation formats. A well known technique is to independently modulate an amplitude of two orthogonal phase components (I and Q phases) of an optical carrier. This is also known as a quadrature amplitude modulation (QAM) having a modulation scheme as shown in FIG. 1. The information content in each phase of a symbol equals the logarithm of base two of the number of amplitude levels. The overall information content equals the sum of the information in both phases.
A further conventional modulation method where the constellation points are located in the I-Q plane is phase modulation. Here, the constellation points are located in a circle around the origin of the plane.
In addition, it is possible and a known technique to modulate an optical carrier which has the same frequency but orthogonal polarization. This conventional method is also called polarization multiplexing.
A measure for the efficiency of a modulation format is the OSNR tolerance of said modulation format for a given bit rate which equals information content per symbol times the symbol rate. With I-Q modulation the best OSNR tolerance is achieved for 4QAM. This OSNR tolerance can be improved by approximately 0.8 dB by using 3-PSK which can be achieved with an I-Q modulator and dependent modulation signals. With conventional polarization multiplexing the bandwidth is reduced by a factor of 2, however twice as much power is required for a constant OSNR and the resulting OSNR tolerance is the same as for single polarization.
Accordingly it is a need to provide a method and an apparatus for modulation of an optical signal which efficiently modulate the optical signal and which provide a high OSNR tolerance.